In a radio communication system, information embedded in electrical signals is broadcasted and intercepted via antennas that convert the electrical signals into and from electromagnetic signals. The power output by an antenna as electromagnetic signals is an important factor in determining the antenna's coverage area or transmission range (i.e., an area within which other devices of the radio communication systems may intercept electromagnetic signals useable for extracting the information therein).
In order to support radio-communications with a large number of devices which are within its transmission range, a base station will typically transmit and receive signals on a number of different channels. Thus, the signals are an overlap of components with different frequencies (i.e., corresponding to different channels). The components (i.e., signals on different channels) may be subject to different gains, while processed and converted into electromagnetic signals. Achieving a uniform gain for all the signal components is desirable for a good transmission quality. The uniform gain may be achieved by pre-distorting the initial signal. Obtaining and maintaining a good transmission quality is based on continuously monitoring and improving the output signal.
The adaptive control in wireless technology often includes accurate measurements of the radio frequency (RF) power output from an antenna of a cellular base-station. Besides enabling to control and regulate the actual RF power level accurately, these measurements may be used for performance measurement, monitoring and supervisory functions (e.g., a voltage standing wave ratio—VSWR—measurement).
It is a challenge to maintain the accuracy of the RF power output measurement over the power range, the frequency range, the operating temperature range and for different waveform characteristics (e.g., different peak-to-average power ratios, constantly changing average amplitudes over the frequency spectrum, varying average and peak power levels with time, etc).
Currently, analog output power measurement methods are frequently used to measure the power output, because analog root-mean-square (RMS) power detector integrated circuits (IC) devices are widely available from several IC component vendors at low cost. The analog RMS power detector IC device outputs a voltage proportional to a measured power, typically extracted from a forward directional coupler at the transmitter RF output. This voltage may be digitized and converted into a measured power value by a software calculation using a response characteristic equation, or using a calibrated look-up-table. Further, a result of a power output measurement may be calculated as an average over many samples.
However, the analog output power measurement accuracy is limited by the precision of converting the voltage into a measured power value, while taking into consideration the fairly high variation of the output voltage due to temperature and frequency. This precision is determined by the complexity and sensitivity of determining the coefficients of the response characteristic equation or the look-up table content. These types of determinations are part of the factory calibration associated with each analog RMS power detector IC device. In this context, cost and time (i.e., duration of the factory calibration) are of essence, limiting the precision. Another limitation affecting analog output power measurement accuracy is the variation of the analog RMS power detector IC device's response for different waveform amplitudes vs. frequency, and amplitude/frequency spectrums vs. time.
An alternative to using analog RMS power detector IC devices is using a digital method in which a portion of the RF power output is processed by analog to digital conversion, quadrature amplitude demodulation, filtering, and measuring power level by calculating the root means square of the sum of squared components, and averaging over multiple samples. Such a method overcomes many of the deficiencies of the analog methods, in particular, the ones regarding reducing the variation in response for frequency and temperature, and removing the variation due to operating power average and waveform characteristics. However, in these methods, additional circuitry becomes necessary to perform down-conversion of the portion of the RF power output to an intermediate frequency, the analog to digital conversion, quadrature amplitude demodulation, etc. This additional circuitry is prohibitive from cost and space point of view.
Methods that reuse existing digital feedback receivers in devices with digitally controlled transmitters may provide a cost effective solution, but an extensive factory calibration and characterization of the involved RF components becomes necessary in order to achieve the desired measurement accuracy. Mapping of RF gains to baseband power readings is difficult from the point of view of the calibration complexity and time, as well as regarding an overall measurement robustness in order to achieve and reliably maintain the required measurement accuracy.
Accordingly, it would be desirable to provide methods and devices for measuring the output power of a device in a radio communication system that avoid the afore-described problems and drawbacks.